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I'm planning on working with a set of non-linear differential equations. A few of the equations look like:

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Mathematica tells me that I is protected. Is there any way I can set $I_{H1}$, for example, as a variable without having to use a different variable to represent it?

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    $\begingroup$ I is the square root of minus 1. Just use another name.... how about "i"? Also, you will be better off not using subscripts as varables... subscripts are a formatting convenience, not a way to define variables. $\endgroup$ – bill s Apr 18 at 3:50
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It is good practice to refrain from naming any variable with an initial capital letter in the ordinary roman font, because such identifiers can clash with ones already defined in the System` context. As substitutes you can use any of the capital letters from the Letters tab of the Special Characters palette which can be display by clicking on Special Characters item on the Palettes menu. For I there are four choices:

alt_cap_I

The first of these looks exactly like ordinary I but it is actually capital iota. Note that it is colored blue, showing that it has no reserved definition.

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